(1) Field of the Invention
The present invention relates to an apparatus for butt welding steel strips by means of a laser beam (hereinafter referred to as "laser butt welding apparatus") in a steel strip-processing line.
(2) Description of the Prior Art
In a steel strip processing line, steel strips are joined together at an entry section of the line, and fed to a latter stage processing and continuously treated and produced. The steel strips have been conventionally joined by a seam welding machine, a flash butt welding machine, or an arc welding machine such as TIG arc welding machine. These welding methods have their own peculiar characteristics respectively. The seam welding method is characterized in that the welding is possible at a high speed in a short time, but the thickness of the welded portion becomes 180-130% of that of the base metal because the steel strips are welded by piling their edges one upon another. Therefore, the welded portion can not be rolled in a coil by taking the problem such as buckling into account. If a scale is present on the surface of the steel strip as in the case of a hot-rolled steel strip or the like, this method has the defect that the surface must be ground prior to the welding. Further, in the case of the welding of thin steel strips, since the strips are pressed by means of electrode wheels, this method has the drawback that the profile of junction and the vicinity thereof are deteriorated.
The flash butt welding method can make the welding time shorter than the seam welding method because the steel strips are welded at one time over their whole width in the former method, but thin steel strips are likely to be bent due to upsetting step, so that this method has the defect that it is applicable to the relatively thick steel strips of not less than 1.6 mm, but is not applicable to the thin steel strips. It also has the defect that it is inapplicable to special steels such as silicon steel, stainless steel, high carbon steel because of oxidation, a large heat input and the coarse grain growth at the welded portion and heat affected zone when the flashing is effected. As a result, the use of flash butt welding is generally restricted to the welding of the low carbon steel strips of not less than 1.6 mm in thickness.
According to the TIG arc welding method, it is possible to improve the quality of the welded portion through addition of a filler wire or the like, and therefore, this method can be used for stainless steel. However, since the heat input is large, it has the defect that it is inapplicable to the material such as the silicon steel in which the coarse grains are likely to grow. Further, it also has the defects that the welding speed is slow, and the welding time is long.
As mentioned above, although the conventional seam welding, flash butt welding, TIG and arc welding methods and the like have been used by utilizing the respective characteristics, they can not be applied to the silicon steel or the like. Thus, there has been a need or demand for a welding apparatus which can weld the special steels such as silicon steel or the like.